I've seen this done mainly on games like Halo, Gmod and a small amount of servers but hey, I think this may be a good idea.

One person is chosen as the fatkid and has an absurd amount of armor and attack damage. However, he moves incredibly slowly and have a low jumping height. He can kill players and have them fight for him (somewhat like infected) and once all are infected, Fatkid wins the game.

The difference between infected and this is that the maps associated with Fatkid give little room for camping. Maps like Dust and Deserted were notorious for catering towards those who could hide rather than those who actually knew how to use their gun. Most of the maps I've seen for Fatkid always wind upwards, eventually ending off where the survivors can run to and force them to turn and fight. These maps also have a few chokepoints but they are often easy to push through so long as the opposing team knows what they are doing.

For this, I'd recommend disabling LMGs as they would end up breaking the gamemode for how GC functions. Additionally, for the Fatkid, I usually see a leap ability toggled to the fatkid's melee to give a slight bit of mobility, The main issues I'd see would be maps, amount of online players and weapon balancing to make sure that the survivors do not have -too- much of an advantage.

LMGs as is would break that gamemode and are often crutches used by people who can't aim. Having 20+ players camping a stairwell, spamming M34s or Negevs (at least) would make it WAY too easy. Better to disable them to make stuff more balanced.